Glass sheets are tempered to provide resistance to breakage. This tempering is performed by first heating the glass sheets to the tempering temperature which is normally a minimum of about 590.degree. C. but preferably 600.degree. to 620.degree. C. or higher without being so hot that the glass sheet becomes too supple to control shape or too soft so as to be easily marked. The heated glass sheet is then suddenly cooled to establish a thermal gradient between the glass surfaces and the center. This thermal gradient is maintained during cooling below the strain point where stresses are essentially permanent, normally about 520.degree. C., and require time periods of minutes for any stress to be relieved. Upon final cooling of the entire glass sheet to ambient temperature, the glass surfaces are in compression and the center is in tension. The surface compression makes the tempered glass sheet more resistant to breakage since most fractures occur in tension at the glass surface and thus must first overcome the compressive stress at the surface before breakage can take place.
Conventionally tempered glass sheets upon breakage fracture into many small fragments that are dull and relatively harmless as opposed to breaking into long shards. Recently, there has been a demand by the architectural industry for glass sheets with resistance to breakage as well as the capability of being maintained within a peripheral frame after breakage as can be done with annealed glass that breaks into shards as opposed to many fragments. Such a glass sheet that is resistant to breakage and does not break into small fragments may also be desirable for vehicle windshields so that the broken windshield will still permit viewing of the roadway so that the vehicle can be operated before the windshield is replaced.
While the ratio of the surface compression to center tension in glass sheets has often been regarded as being more or less fixed at about 2.0 for tempered glass, values of 2.6 have been experimentally obtained with air tempering but require that the glass be heated substantially higher than the normal tempering temperature. Also, the surface compression and center tension are both relatively high with such higher values of the ratio such that the glass dices upon breakage due to its high center tension. Thus, merely tempering glass with a high surface compression to center tension ratio without individually controlling surface compression and center tension is of less value than when those stresses can be individually controlled.